For any dedicated chili-lover, Thai restaurant "spice scales" are always a huge disappointment -- and a baffling mystery.

Usually marked with bright red pepper symbols on ethnic menus, the heat levels (mild, medium, hot) seem to vary from restaurant to restaurant and sometimes even cook to cook in the same kitchen.

The system might work well for the pepper-sensitive -- a zero on any scale is bound to be mayonnaise-bland instead of tongue-tingling -- but for those of us who crave culinary heat, even extra hot can barely tickle our jaded taste buds.

We can beg the chefs, we can plead with servers, but endorphin addicts know that it's nigh on impossible to induce most Thai chefs in America to spoon up the hot stuff. Until recently, I figured Simply Thai was another such place -- bound and determined to coddle my tongue.

I've always liked Simply Thai. It's a cute little spot with lavender walls, minimal decorations and inside seating for a few dozen at the most (and barely enough outdoor seating to accommodate the overflow on busy nights).

When it comes to artful presentation, nuance and delicacy, Simply Thai is the best Louisville restaurant in its category. Edamame dumplings ($4.50) are a case in point: Resting on a white plate, they look like jade-green plants poking their tips through a cover of snow. They're tender little morsels filled with gleaming green soybeans and flavored with mellow fish paste. And their house-made dipping sauce -- a garlicky, gingery, sweet and salty concoction -- is flavorful enough to drink by the bowlful.

Those dual themes of beautiful design and rigorous execution recur across the menu -- from structurally sound spring rolls ($4.50) to such classic noodle dishes as pad Thai (chicken, tofu, $8; shrimp, $10; nearly every dish on the menu can be prepared to vegan or vegetarian standards); from lemongrass-scented tom yum soup ($5) to beautifully grilled slices of pork loin served with fragrant cilantro rice ($14).

But as for heat? The servers routinely ask customers to state their preference on a scale of 0-5. But that scale has never worked for me. Even when I've specified the highest number and urged a server to be bold, I've never encountered more than a faint tickle of heat that hardly cuts through the kitchen's bias toward slightly sweet flavors.

Then, not long ago, I overheard a customer at a nearby table do something utterly bold and -- to me, at least -- unthinkable.

He said, "Six."

Suddenly, in a flash of insight, I saw the truth: The 5 on Simply Thai's scale wasn't the end of the line; it was just a mile marker on the road to ecstasy.

A few days later, I asked for a bowl of green curry ($10) at the 7 level. The green curry looked pretty standard: creamy, pale green and generously studded with tender chunks of chicken breast and crisp green beans. But now the sauce packed enough peppery punch to burn my lips and give me a serious case of the sniffles.

Likewise, a plate of "loaded" fried rice ($8/$10) demonstrated a pleasantly incendiary edge. Jasmine rice was mixed with black beans, bright green edamame, an assortment of vegetables and lovely caramelized onion strips -- but was also tinged red by enough finely minced chilis to numb the lips -- but not enough to obscure the full range of flavors lurking beneath.

Next time, I'll ramp it up to 8. And after that -- who knows? -- I may go full "Spinal Tap" and crank it all the way to 11.